Simplex noise
A smooth noise built on a triangle grid, quicker than Perlin noise and free of its grid lines.
See it in Noise & texture. Set the noise type to Simplex, then compare it with Perlin.
In plain words
Smooth noise is built by placing random values on a grid and blending between them. Perlin noise uses a square grid, which can leave faint lines along the rows. Simplex noise uses triangles instead. That hides the grid and needs fewer sums, so it runs faster. It was made by the same person who made Perlin noise, years later.